1. Field of the Invention
This invention relates to a prosthetic device implanted in vertebrae for treatment or prevention of back pain in patients with ruptured or degenerated intervetebral discs.
2. Description of the Prior Art
The spine of human is constituted of backbone and fibrocartilaginous disc. FIG. 1 shows a lumbar (200) of human, which consists of vertebral body (220), two pedicles (210) and two intervertebral discs (230). The intervertebral disc (230) is frequently referred to as xe2x80x9cdiscxe2x80x9d and consists of annulus fibrous and nucleus pulpous.
Low back pain is known to be caused by collapse of the disc (230) and adverse effects of bearing the majority of the body weight through a damaged unstable vertebral joint. In the past, clinical attention has been focused on relief of a sciatic pain by removal of pressure from a nerve root. This kind of surgical treatment falls into the following groups.
A 1st method is xe2x80x9cExcision of the Ruptured Soft Discxe2x80x9d. This procedure removes a portion of the disc compressing the spinal nerve and is generally successful in relieving the sciatic leg pain but in more than half the cases, there is a recurrence of back pain. Over a period of time the disc gradually loses height due to the rupture and said loss of height causes the posterior facet joints of the vertebrae to fit incorrectly, resulting in arthritic change in all elements of the spinal segment.
A 2nd method is xe2x80x9cDisc Excision With Posterior Fusionxe2x80x9d. It prevents motion between adjacent vertebrae but does not alter the fact that approximately 90% of the body weight must be transmitted through degenerated discs, causing pain. Further, posterior fusion tends to bring about bony overgrowth, leading to nerve root compression by spinal stenosis.
A 3rd method is xe2x80x9cDisc Excision With Anterior Interbody Fusionxe2x80x9d. In this method, a soft disc is completely excised and replaced with either a patient""s own bone (autologous bone) or with transplant. Banked bones (homologous bone) are generally successful if solid fusion can be obtained between adjacent vertebrae bodies, The success rate has been only about 50%.
A 4th method is xe2x80x9cDisc Excision With Posterior Lumbar Intervertebral Fusion (PLIF)xe2x80x9d. This procedure reconstructs a normal anatomic relationship between the bony and the neural structures and has many advantages. However, this procedure has several serious disadvantages in that it is technically very difficult, and is therefore not as successful or widely used as it might be.
In the art of spinal surgery, various devices and methods for interbody fusion have been developed and are described in U.S. Pat. No. 5,772,661 issued Jun. 30, 1998 to Michelson. U.S. Pat. No. 5,683,391 issued Nov. 4, 1997 to Lawrence M. Boyd describes a system for attachment of cylindrical interbody fusion devices to a spinal rod to which bone screws are also attached and anchored in the vertebral bodies. It is desirable that an interbody fusion construct be as stable as possible. Also, it is desirable to use an endoscopic procedure, if possible. U.S. Pat. No.6,156,037 issued Dec. 5, 2000 is a system for achieving these benefits.
It discloses an interbody fusion cage having an externally threaded stem (119) projecting from a domed outer end (122). In this invention, a contoured plate is provided with an aperture receivable on the stem. The stem threads receive a nut (121) to fix the plate (111) to a cage (112). A plate (111) has additional apertures receiving bone screws anchoring the plate to vertebral bodies (113,114). The plate (111) has a hemispherical surface surrounding the stem-receiving aperture and bearing on the dome, accommodating universal angulation of the plate relative to the cage.
However, in such devices with an externally threaded cylindrical body, there was a concern in that the body could slide backward or forward to make a fixed state become unstable, resulting in requirement for complicated fixing device.
Therefore, it is an object of this invention to provide a device for overcoming the above-mentioned problem without recourse to complicated device.
According to one feature of the invention, a prosthetic cage for spine, which is adapted to be implanted in the intervertebral disc space between two vertebral bodies in a spine and to accommodate fusion of the disc space, is a tubular body with an internal aperture, and its external surface has a plurality of wall apertures each perpendicular to the longitudinal axis thereof and communicating with the internal aperture, and dual convex screw threads extended therefrom.
According to another feature of the invention, the prosthetic cage for spine has dual convex screw threads, the inclined planes of which are oppositely directed from each other.
According to still another feature of the invention, the prosthetic cage for spine has dual convex screw threads that are spaced apart from each other and wall apertures are available also between the dual convex screw threads.
According to still another feature of the invention, an introduction end portion of the cage has a sharpened section for stimulating an intervertebral disc space.